This invention relates generally to the purification of vinyl olefins and more particularly to the selective removal of branched chain olefin and especially vinylidene olefin impurities by converting them to oligomers which are easily separated from the vinyl olefins.
Olefin mixtures containing vinyl, vinylidene and internal olefins of similar carbon number are difficult to separate by distillation because they boil very close together. Such mixtures generally result when the olefins are made by a process capable of producing all three types of olefins. For example, the ethylene chain growth process using triethylaluminum followed by olefin displacement. The olefin type produced is mainly vinyl olefins, i.e. R--CH.dbd.CH.sub.2, where R is an aliphatic hydrocarbon group, but the product also contains lesser amounts of internal olefins, i.e. R--CH.dbd.CH--R, where R is an aliphatic hydrocarbon group, and vinylidene olefins, i.e. ##STR1## where R and R' are aliphatic hydrocarbon groups. When practiced to produce olefin mixtures containing up to 12 carbon atoms, the mixtures are predominantly, i.e. about 80 mole percent or more vinyl olefins. However when practiced to produce higher olefins, e.g. containing 14 or more carbon atoms, the amount of internal olefins, and especially vinylidene olefins, increases sharply such that in the C.sub.16-18 olefin range the olefin mixture will contain about 20 to 55 mole percent vinylidene olefins and 5 to 20 mole percent internal olefins. In some uses the vinylidene olefin content of the olefin mixtures is not detrimental. However, in some uses the presence of vinylidene olefin decreases the value of the olefin mixture. For example, detergents can be made by reacting olefin mixtures with hydrogen sulfide to add hydrogen sulfide to the double bond forming a mercaptan. These in turn can be oxidized to form sulfonic acids which when converted to their salts are effective detergents. However, vinylidene olefins react with hydrogen sulfide to form tertiary mercaptans which are very difficult to oxidize to sulfonic acids. Thus, a need exists for a process for separating vinylidene olefins from a mixture containing vinyl, vinylidene and internal olefins which mixtures cannot be readily separated by distillation.
It has now been discovered that olefin mixtures containing vinyl, vinylidene and internal olefins can be upgraded to lower the vinylidene olefin content by reacting the mixture using a BF.sub.3 -promoter catalyst system to preferentially dimerize the vinylidene olefins. The dimerized product is not only easily separated from the product mixture by distillation, but the product, especially after hydrogenation to remove residual unsaturation, is a useful synthetic oil such that the purification process is very economical.